Anode target for X-ray tube and X-ray tube therewith

ABSTRACT

An X-ray tube which is high in brightness and high in resolution, and can withstand continuous long-time use, that is, it can withstand a high heat load. An X-ray target and an X-ray tube having the X-ray target include an X-ray generating metal layer having an average crystal grain diameter not larger than 30 μm on the surface of a base plate in the X-ray irradiated side. The X-ray tube has a small focus point and can withstand a high input load. A CT apparatus using the X-ray tube can provide a high resolution and a high definition image.

TECHNICAL FIELD

The present invention relates to an X-ray tube generating an X-ray byirradiating an electron beam, an anode of an X-ray target of an X-raytube and an X-ray apparatus using the X-ray tube and, more particularly,to a medical X-ray tube and a medical X-ray apparatus which is requiredto be high in load resistivity and high in brightness and definition ofan image.

BACKGROUND ART

In an X-ray generating apparatus for industrial use or medical use, anX-ray is generated by irradiating thermal electrons emitted from ancathode onto an anode target. An X-ray generating metal for the anodetarget (hereinafter, referred to as “X-ray target”) used is tungsten (W)or a tungsten alloy which has a high X-ray generating efficiency and ahigh melting point.

An X-ray tube for medical use is required to produce a high definitionimage of a medical examination portion and to have a higher X-ray outputcompared to a common X-ray tube. Since most part of energy of anelectron beam is converted into heat when an X-ray is generated, theX-ray target is heated to high temperature.

Further, a high power X-ray tube is so constructed that the X-ray targetis rotated during electron beam irradiation in order to prevent theX-ray target from overheating. Therefore, the X-ray tube is required tohave a high heat resistance and a high strength during rotation. Amethod for coping with this problem is disclosed, for example, inJapanese Patent Application Laid-Open No.58-59545. In the method, atungsten or tungsten alloy layer is formed onto the surface of amolybdenum or molybdenum alloy base plate through a chemical depositionmethod or the like. This method has an advantage in better bondingability between the surface of the molybdenum alloy base plate and thetungsten alloy layer and accordingly in a high thermal conductivity. Amethod of manufacturing an X-ray target is also disclosed in JapanesePatent Application Laid-Open No. 57-176654. In the method, a tungsten ortungsten alloy layer is successively laminated onto the surface of amolybdenum or molybdenum alloy base plate through a chemical depositionmethod or the like, and then the laminated X-ray target is annealed toimprove the adhesive force. The X-ray tubes using such X-ray targetshave a better load resistivity compared to an X-ray tube having aconventional X-ray generating metal, and can withstand a longtime andcontinuous use.

As the progress of an X-ray apparatus with computer processing such as aX-ray CT apparatus for medical use, an X-ray tube is required to copewith a high resolution processed image. Further, it is required that theX-ray tube can withstand a long-time and continuous use. In order to doso, it is necessary to increase input power to the X-ray tube toincrease the amount of X-ray radiation. In addition to this, in order toobtain a high resolution image, it is important to converge an electronbeam from a cathode small, that is, to increase the brightness by smallfocusing and large current density. Therefore, it is required that theX-ray target can withstand a large heat load on the electron irradiationsurface. To these requirements, the method of Japanese PatentApplication Laid-Open No.58-59545 has a problem in that the surface ofthe X-ray generating metal made of a tungsten alloy is roughed and theX-ray generating efficiency is decreased as it is used long time.

On the other hand, the method of Japanese Patent Application Laid-OpenNo.57-176654 has a disadvantage in that the process of manufacturing thetarget is complex and accordingly its manufacturing cost may beincreased.

DISCLOSURE OF INVENTION

An object of the present invention is to provide an X-ray tube which ishigh in brightness and high in resolution, and can withstand continuouslong-time use, that is, can withstand a high heat load, and to providean X-ray apparatus such as an X-ray CT apparatus capable of obtaining amore clear image using the X-ray tube.

The object of the present invention can be attained by providing anX-ray tube generating an X-ray from a metal surface by irradiating anelectron beam, wherein at least a part of an electron irradiatingsurface of an anode target of the X-ray tube comprises an X-raygenerating metal having an average crystal grain diameter not largerthan 30 μm, preferably not larger than 10 μm, on the surface of a baseplate made of a metal. The “average crystal diameter” here means a minoraxis when the crystal grain is flat. The crystal grain diameter may beobtained by taking a picture of a polished surface using an opticalmicroscope or an electron microscope, and calculating through an imageprocessing method or measuring crystallographically using an X-ray. Inthese cases, although the crystal grain diameter is apt to be measuredsmaller in a case of using the X-ray, it is sufficient that the measuredaverage crystal grain diameter is within the above range whichevermethod is chosen.

It is preferable that the X-ray generating metal having an averagecrystal grain diameter not larger than 30 μm is composed of two or morelayers. The “two or more layers” means that the composition of eachlayer may be different, or a boundary may be simply formed betweenlayers. For example, in a case of forming an X-ray generating metallayer through the chemical vapor deposition method, by stopping tosupply the process gas for a while during forming a layer and thenstarting to supply the process gas, a boundary is formed and two layerscan be observed. In film forming through chemical vapor deposition, seedcrystals are firstly formed on a base plate and then crystals grow basedon the seed crystals to form a film. When supply of the process gas isstopped for a while, crystal growth is stopped at that time. When supplyof the process gas is started again, seed crystals are newly formed. Insuch a way, two or more layers of metal films can be formed even if thecomposition of each of the layers is the same. The most convenient wayto judge whether two or more layers are formed is to polish a crosssection of the X-ray target and observe it by a microscope.

Further, it is preferable that, in the X-ray tube, the X-ray generatingmetal having an average crystal grain diameter not larger than 30 μm iscomposed of two or more layers containing tungsten and rhenium, andtungsten concentration in the layer in contact with the metal base plateis higher than tungsten concentration in the surface layer of theelectron irradiating surface. A preferable X-ray generating metal is asubstance having a larger atomic number which has a higher X-raygenerating efficiency, but it is required to have a higher meltingpoint. Although tungsten is generally used as an element to satisfythese requirements, rhenium is added as an alloy element since tungstenitself is low in high temperature strength and accordingly is unsuitablefor practical use.

It is also preferable that the thickness of the X-ray generating metallayer is not larger than 200 μm.

It is preferable that the X-ray generating metal layer described abovehas a tungsten alloy layer in the side of the base plate.

Further, the present invention provides an X-ray tube in which at leasta part of an electron irradiating surface of an anode target of theX-ray tube comprises two or more layers of alloy layers on the surfaceof a metal base plate. The definition of “two or more layers” is thesame as described above.

Furthermore, the present invention provides an X-ray tube generating anX-ray from a metal surface by irradiating an electron beam in which atleast a part of an electron irradiating surface of an anode target ofthe X-ray tube comprises an X-ray generating layer having a columnarcrystal structure on the surface of a metal base plate. The “columnarcrystal structure” hear means a crystal structure in which directions ofcrystals (directions of longitudinal axis of the crystals) are orientedin nearly the same direction and the aspect ratio of the crystal isapproximately more than 5.

Further, the present invention provides an X-ray tube generating anX-ray from a metal surface by irradiating an electron beam, in which atleast a part of an electron irradiating surface of an anode target ofthe X-ray tube comprises an X-ray generating layer made of tungsten andrhenium on the surface of a metal base plate, and concentration ofelements except for the tungsten and the rhenium in the X-ray generatingmetal is not larger than 100 ppm. The concentration is indicated by unitof weight ratio and analyzed through a method such as chemical analysis,instrumental analysis or the like.

It is preferable that the metal layer containing tungsten and rheniumhaving maximum thickness of not larger than 100 μm is formed at least ona part of a base plate made of a metallic sintered material havingmolybdenum as the main component in the side of electron irradiatingsurface. There is no need that the X-ray generating metal layer coversthe whole surface of the electron irradiating surface of the metal baseplate, but the X-ray generating metal layer may exist in, for example, aradial shape. It is preferable that a metal layer containing tungstenand rhenium having an average crystal grain diameter not smaller than 30μm is formed at least on a part of a base plate made of a metallicsintered material having molybdenum as the main component in the side ofelectron irradiating surface, and the metal layer having average crystalgrain diameter not larger than 10 μm is formed at least on a part of themetal surface having an average crystal grain diameter not smaller than30 μm in the side of electron irradiating surface. It is preferable thata clear boundary exists between the metal surface having an averagecrystal grain diameter not smaller than 30 μm and the metal layer havingaverage crystal grain diameter not larger than 10 μm.

Further, it is preferable that the metal layer containing tungsten andrhenium is formed at least on a part of a base plate made of a metallicsintered material having molybdenum as the main component in the side ofelectron irradiating surface, and distribution of rhenium in the metallayer is uniform. When a cross section of an X-ray generating metal of asintered material sintered formed by adding rhenium powder is observedby a scanning electron microscope and analyzed by an electron probemicro-analyzer, it is found that rhenium particles as it is exist in thesintered material and accordingly there is deviation in rheniumdistribution. In a case of forming the metal film through a method suchas chemical vapor deposition method, physical vapor deposition method,sputtering method or the like, such variation does not exist and rheniumis uniformly dispersed in the tungsten.

It is preferable that the metal layer containing tungsten and rhenium isformed at least on a part of a base plate made of a metallic sinteredsubstance having molybdenum as the main component in the side ofelectron irradiating surface, and relative density to the theoreticaldensity of the metal layer is not smaller than 98%. A value described ina chemical handbook or the like is used as the theoretical density. Thedensity may be measured through a hydraulic replacing method(Archimedes' method) or the like. The most convenient way to measure thedensity of the X-ray generating metal of metal thin film is tomechanically peel off the film from the base plate.

It is preferable that the composition ratio of rhenium to tungsten ofthe metal layer containing tungsten and rhenium is larger in theelectron irradiated side of said layer. The efficiency of generatingX-ray is larger in a metal having a larger atomic number. The atomicnumber of tungsten is 74 and the atomic number of rhenium is 75.Therefore, the efficiency of generating X-ray is larger in rhenium thanin tungsten. On the other hand, the penetrating depth of electron intothe X-ray generating metal surface is approximately 10 μm, but itdepends on the energy of electron. Therefore, it is preferable that thecontent of rhenium is made large in the zone up to the depth of 10 μmfrom the surface and the content of tungsten is increased as the depthapproaches to the metal base plate. The melting point of rhenium islower compared to that of tungsten, and the price of rhenium is highercompared to that of tungsten. In regard to surface melt and cost, it isnot preferable to make the content of rhenium excessively high.

FIG. 1 is a view showing a simulation result of temperature distributionin an X-ray target of an X-ray tube during using. Temperature at thesurface of the electron irradiating surface is increased up toapproximately 1500° C., but temperature at a position beneath thesurface is steeply decreased. In a case where graphite is used as thebase plate and an X-ray generating metal layer is formed on the electronirradiating surface though chemical vapor deposition method, temperatureat the boundary between the graphite base plate and the X-ray generatingmetal layer is increased above 1300° C. since the X-ray generating metallayer is formed so as to have a thickness less than 500 μm due tomanufacturing cost. In such a temperature condition, the graphite reactswith the tungsten in the X-ray generating metal layer made of atungsten-rhenium alloy to form a carbide such as tungsten carbide. Whensuch a carbide is formed, the bonding force in the boundary isdecreased, and cracks and delamination possibly occur at the junctionportion during using the X-ray tube.

Since such a carbide has a small thermal conductivity, the heatgenerated on the electron irradiating surface is not sufficientlydispersed. That is, the temperature of the electron irradiating surfaceis increased and the load resistivity is decreased.

The inventors of the present invention invented the present invention bystudying an X-ray target which did not decrease its load resistivity dueto formation of such a carbide. That is, the inventors of the presentinvention found that an X-ray target having a high load resistivitycould be obtained by making the base plate of the X-ray target with ametal sintered material such as molybdenum and forming an X-raygenerating metal film having average grain diameter smaller than 30 μmon the base plate using a thin film technology such as a chemical vapordeposition method.

There is a phenomenon that the surface shape of the X-ray generatingmetal is roughened when an X-ray tube is used for long time. Thisphenomenon is caused by sublimation or melting of the X-ray generatingmetal because the temperature near the electron irradiating surfaceincreases up to approximately 2000° C. When the surface is roughened,the X-ray generating amount is decreased because X-ray emitted from thesurface of the X-ray generating surface is scattered by the roughsurface. FIG. 2 is a schematic view showing this phenomenon.

The inventors found that small crystal grain diameter was effective tosuppress this phenomenon. The reason is that sublimation and melting ofthe X-ray generating surface occur in the grain boundaries first. FIG. 3is a schematic view showing this phenomenon.

From these facts, the inventors found that an X-ray tube had a highbrightness and a small degradation in performance when it was used for along time. The X-ray tube comprised an X-ray target of an X-raygenerating metal layer having average grain diameter not larger than 30μm, preferably not larger than 10 μm, formed through chemical vapordeposition method or the like.

FIG. 4 is a graph showing the relationship between crystal graindiameter and surface roughness of an X-ray generating metal layer. Inorder to accelerate testing time, this test was performed by irradiatingYAG laser instead of electron beam to supply a high heat input andmeasuring worn amount of the X-ray generating metal surface. It can beunderstood from the result that the X-ray target having a crystal graindiameter smaller than 10 μm is smaller in worn cross sectional area andsmaller in surface roughness than the X-ray target having a crystalgrain diameter of nearly 50 μm. The reference character Z in FIG. 4indicates the distance between the center of a laser focus lens and asample surface. FIG. 5 is photographs showing cross-sectional features.The photograph in FIG. 5(a) shows a cross-sectional feature of thechemical vapor deposited tungsten-rhenium layer (20 go-and-returncycles), and the photograph in FIG. 5(b) shows a cross-sectional featureof the sintered tungsten-rhenium layer (20 -go-and-return cycles). Thelength of 1 cm in FIG. 5 corresponds to 20 μm.

FIG. 6 is a graph showing dependence of crystal grain diameter in X-raygenerating metal on heating temperature. It can be understood that thecrystal grain diameter of an X-ray generating metal layer having initialgrain diameter of nearly 1 μm is grown not so large after heating at2000° C. for 1 hour. This means that the crystal grain diameter of theX-ray generating metal layer does not coarsen with time and accordinglythere is little problem in surface roughing.

An X-ray target shown in FIG. 7 was manufactured. The X-ray target wasmanufactured by forming a tungsten-rhenium sintered alloy havingthickness of approximately 10 μm on the surface of a molybdenum sinteredalloy base plate to manufacture a base X-ray target, and by furtherforming an X-ray generating metal layer having crystal grain diametersmaller than 10 μm and thickness of 100 μm on the half surface of thebase X-ray target. The X-ray target was irradiated with an electron beamfor a predetermined cycles while the X-ray target was being rotated, andthen rotation of the target was stopped. FIG. 8 is a graph showing themeasured result of amount of generated X-ray and reducing ratio of X-raygeneration for the side with the X-ray generating metal layer and theside without the X-ray generating metal layer. The amount of generatedX-ray is more in the side with the X-ray generating metal layer bynearly 10% than in the side without the X-ray generating metal layer.The reducing ratio of generated X-ray is less in the side with the X-raygenerating metal layer by nearly 5% than in the side without the X-raygenerating metal layer. FIG. 9 is photographs showing cross-sectionalstructures near the X-lay generating metal layers after the test. Thephotograph in FIG. 9(a) shows a cross-sectional feature of the chemicalvapor deposited tungsten-rhenium layer, and the photograph in FIG. 9(b)shows a cross-sectional feature of the sintered tungsten-rhenium layer.The length of 1 cm in FIG. 9 corresponds to 100 μm. The surfaceroughness is smaller in the side with the X-ray generating metal layerthan in the side without the X-ray generating metal layer. Measurementby a probe type surface roughness meter showed that the averageroughness (Ra) and the maximum roughness (Rmax) in the side with theX-ray generating metal layer were 5.7 μm and 45 μm, and on the otherhand the average roughness (Ra) and the maximum roughness (Rmax) in theside without the X-ray generating metal layer were 7.5 μm and 71 μm.That is, the surface roughness was smaller in the side with the X-raygenerating metal layer than in the side without the X-ray generatingmetal layer.

After studying the differences in the test results of the X-ray targetwith the X-ray generating metal layer and the X-ray target without theX-ray generating metal layer, the following results are obtained.

(1) When the crystal grain diameter of the electron irradiating surfaceis smaller than a certain value, the surface roughness is small.

(2) When there is a boundary between the surface layer and the baseplate, a crack starting from a point on the surface is suppressed toprogress and the crack progress distance is shortened.

(3) It is revealed from an analysis using an electron probemicro-analyzer that rhenium distribution in the X-ray generating metallayer formed on the surface is uniform compared to that in the sinteredtungsten-rhenium layer.

(4) The relative density to the theoretical density is large in thesurface of the X-ray generating metal layer than in the surface of thesintered tungsten-rhenium layer. That is, the sintered tungsten-rheniumlayer has a lot of voids and the surface roughness is large.

Based on the above test data, the requirements for an X-ray tube havinghigh brightness and long life-time are obtained as follows.

(1) An X-ray generating metal layer having a maximum drain diameter notlarger than 30 μm, preferably a maximum grain diameter not larger than10 μm, is formed on the surface of a metal base plate made of molybdenumor the like.

(2) A boundary exists between the X-ray generating metal layer and themetal base plate or inside the X-ray generating metal layer to preventprogress of a crack.

(3) Rhenium distribution in the X-ray generating metal layer is uniform.

(4) Relative density to the theoretical density in the X-ray generatingmetal layer is not smaller than 98%.

With the above specified construction, an X-ray tube having highbrightness and long life-time can be obtained.

A method of manufacturing an X-ray generating metal layer in accordancewith the present invention is characterized by that a tungsten-rheniumfilm of the X-ray generating metal is formed by using metal halide gases(WF₆, ReF₆) containing hydrogen and maintaining the base platetemperature within the range of 200 to 600° C., preferably 400 to 500°C., in which the film forming speed is high and a uniform fine structurecan be obtained. When the base plate temperature is lower than 200° C.,the film is apt to become non-uniform. On the other hand, when the baseplate temperature is higher than 600° C., the fine structure is hardlyobtained because content of rhenium becomes low. In order to make thefilm forming speed high, it is preferable that the chemical vapordeposition pressure is set to near atmospheric pressure. Further, it isalso preferable that an amount of rhenium contained in the finestructure tungsten-rhenium alloy is in the range of 2.5 to 26 wt % inorder to form the fine structure.

As for a method of manufacturing an X-ray target in accordance with thepresent invention, it is preferable that a fine structuretungsten-rhenium alloy as an x-ray generating metal material is coatedonto a heat resistant anode base plate made of molybdenum or amolybdenum alloy, or tungsten or a tungsten alloy, or a complex baseplate formed by laminating layers made of the materials, and then thecoated X-ray target is performed with heat-treating at a temperature of1000 to 2000° C. in a vacuum environment. By the vacuum heat treatment,diffusion between the metal base plate and the X-ray generating metalcoated onto the metal base plate is progressed, and at the same time gascontained in the X-ray target is completely removed. When the heatingtemperature is lower than 1000° C., diffusion between the coated X-raygenerating metal and the base plate made of molybdenum or the molybdenumalloy, or tungsten or the tungsten alloy, or the complex base plateformed by laminating layers made of the materials is insufficient andaccordingly the coated X-ray generating metal cannot closely attached tothe base plate or the complex base plate. Further, the degassing of theX-ray target is insufficient and accordingly the withstanding voltage islowered due to gas released when the X-ray target is assembled in anX-ray tube. Therefore, an X-ray having a sufficient strength cannot begenerated.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view showing a simulation result of temperature distributionin an X-ray target.

FIG. 2 is a schematic view showing X-ray scattering on the surface of anX-ray target.

FIG. 3 is a schematic view showing crystal grain diameters androughnesses of X-ray generating metal surfaces.

FIG. 4 is a graph showing results of laser acceleration test of X-raygenerating metals.

FIG. 5 is photographs showing cross-sectional features after the laseracceleration test.

FIG. 6 is a graph showing the relationship between heating temperatureand crystal grain diameter in X-ray generating metal of an X-ray targetin accordance with the present invention.

FIG. 7 is a view showing an X-ray target of which half-circle surface iscovered with an X-ray generating metal in accordance with the presentinvention.

FIG. 8 is a graph showing reducing ratio of X-ray generation and amountof X-ray generation of X-ray targets after an actual load test.

FIG. 9 is photographs showing cross-sectional structures after an actualload test.

FIG. 10 is a cross-sectional view showing the construction of an X-raytube having an X-ray target in accordance with the present invention.

FIG. 11 is a cross-sectional view showing the construction of anembodiment of an X-ray target in accordance with the present invention.

FIG. 12 is a photograph showing the surface appearance of an X-raytarget in accordance with the present invention after an actual loadtest.

FIG. 13 is a photograph showing the surface appearance of a conventionalX-ray target after an actual load test.

FIG. 14 is a cross-sectional view showing the construction of anotherembodiment of an X-ray target in accordance with the present invention.

FIG. 15 is a cross-sectional view showing the construction of anotherembodiment of an X-ray target in accordance with the present invention.

FIG. 16 is a cross-sectional view showing crystal structure of an X-raytarget in accordance with the present invention after a heating test.

FIG. 17 is a schematic view showing the multi-layer structure of anotherembodiment of an X-ray target in accordance with the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

(Embodiment 1)

FIG. 10 is a schematic cross-sectional view showing an embodiment of anX-ray tube having an X-ray target manufactured through a method inaccordance with the present invention.

An X-ray tube 10 contains an X-ray bulb 100 inside an enclosingcontainer 11. A coolant 15 is filled around the X-ray bulb globe 100 inthe enclosing container. The enclosing container 11 has an X-rayradiating window 12. The X-ray radiating window 12 preferably has a leadslit constructed, for example, by attaching lead plate onto the outersurface or onto the inner surface of a glass plate except for a portionthrough which an X-ray is emitted. It is also preferable that an X-rayshielding member, for example, a lead plate is attached onto the innersurface of the closing container in addition to the X-ray radiatingwindow.

The X-ray tube generates an abundance of heat as well as radiation ofX-ray. In order to forcibly cool the generated heat, the coolant 15 isfilled inside the closing container and recirculated. The coolant filledis preferably a liquid, for example, an insulating oil.

The X-ray bulb 100 has a rotating anode 120 and a cathode 130 in avacuum outer enclosure 110. The vacuum outer enclosure 110 is made ofglass or a complex material of metal and glass. The rotating anode 120has an X-ray target 121 and a rotating mechanism for the X-ray target.The rotating mechanism for X-ray target has a motor rotor. A motorstator 125 is provided in a position outside the X-ray tube facing therotor.

The cathode 130 has a filament for emitting an electron beam, and theemitted electron beam 131 is irradiated onto the X-ray target 121, andthe emitted X-ray is released through the X-ray radiating window 12 ofthe closing container 11. The reference character 129 indicates an anodeterminal, and the reference character 139 indicates a cathode terminal.The reference characters 141, 142 indicate parts for containing andfixing the X-ray bulb 100 inside the closing container 11. The referencecharacter 111 indicates a vacuum sealing portion for evacuating theinside of the vacuum outer enclosure 110 and its end is finally sealed.

In FIG. 10, a rubber cap 13 is placed on the top end of the closingcontainer 11. The rubber cap is provided for cope with the volume changeof the insulating oil due to temperature rise of the X-ray bulb and theinsulating oil by operation of the X-ray bulb. The rubber cap 13 preventthe coolant from flowing out due to pressure rise by utilizing expansionand contraction action of rubber.

The X-ray target in accordance with the present invention is suitablefor using as a rotating anode in the X-ray tube having the constructionshown in FIG. 10. Further, the X-ray target in accordance with thepresent invention is suitable for a small focus point and high brightX-ray bulb since it can withstand a large heat load.

An X-ray target having a cross-sectional construction shown in FIG. 11is employed as an anode target of a X-ray tube as described above. Acenter hole 7 is a hole for introducing a rotating shaft (not shown)made of molybdenum, and the X-ray target and the rotating shaft arefastened by a nut (not shown) or the like made of molybdenum. Further, asloped portion for extracting X-ray is provided on the circularperiphery of the X-ray target. The base plate has a construction ofsintered tungsten-rhenium/molybdenum/graphite formed by bonding graphite4 onto the electron non-irradiated surface side of the metal target 8using a high melting point metal solder 5, and an X-ray generating metalof a fine structure tungsten-rhenium alloy 6 is coated on a sinteredtungsten-rhenium alloy 1 having a rough crystal diameter to be used asan electron irradiating surface of the 5 inch diameter base platethrough chemical vapor deposition method. The chemical vapor depositionis performed by heating the base plate at 450° C. in a hydrogen gasenvironment, and then introducing a mixed gas containing WF₆ and ReF₆ onthe base plate. The base plate except the electron irradiating surfaceis masked with a graphite mask and the base plate is rotated with nearly10 rpm during performing vapor deposition in order to uniformly coatingthe circular periphery of the base plate. The prototype X-ray target isperformed with vacuum heat treatment at 1400° C. for 1 hour. The graindiameter of the fine structure tungsten-rhenium alloy at that time is0.9 to 4.5 μm. Then, the target is assembled into a rotating anode andvacuum-sealed in an X-ray tube having a structure shown in FIG. 10. Anactual load test was conducted using the above X-ray tube. Aftergenerating 50000 shots of X-ray under condition of tube voltage of 120kV and tube current of 400 mA, change in the X-ray generating amount wasinvestigated. The X-ray generating amount decreased compared to in theinitial stage since the surface of the X-ray target was roughed due toirradiation of electron beam. The decreasing ratio of X-ray generatingamount of the X-ray target coated with the fine structuretungsten-rhenium alloy in accordance with the present invention wasapproximately 5%. The decreasing ratio of X-ray generating amount of theconventional X-ray target not coated with the fine structuretungsten-rhenium alloy was approximately 15% compared to the initialvalue. The X-ray tube in accordance with the present invention was smallin decreasing amount of X-ray generation and the high load resistibilitywas obtained. The surface of the X-ray target after actual load test waspolished and heat cracks were observed. FIG. 12 is a photograph showingheat cracks in the X-ray target in accordance with the presentinvention, and FIG. 13 is a photograph showing heat cracks in theconventional X-ray target. The heat cracks in the X-ray target inaccordance with the present invention are very fine. Length of 1 cm inFIG. 12 and FIG. 13 corresponds to 100 μm.

(Embodiment 2)

FIG. 14 is a cross-sectional view showing the construction of anotherembodiment of an X-ray target in accordance with the present invention.The X-ray target is a metal target in which a sintered tungsten-rheniumalloy 1 having a coarse crystal grain diameter is laminated onto amolybdenum base plate 2. The base plate has a mixed oxide coating layer3 containing titanium, zirconium, aluminum and so on formed onto theelectron non-irradiating surface through a melt spray method to increaseits thermal radiation. The base plate is coated with a finetungsten-rhenium alloy through the chemical vapor deposition method asthe same manner as in Embodiment 1. Then the mixed oxide coating layer 3containing titanium, zirconium, aluminum and so on is formed onto theelectron non-irradiating surface through a melt spray method. The targetis performed with vacuum heat treatment and is vacuum sealed in an X-raytube as the same as in Embodiment 1. An actual load test was conductedusing the above X-ray tube. As the result, the same performance as inEmbodiment 1 was obtained.

(Embodiment 3)

A fine structure tungsten-rhenium alloy is coated onto the same baseplate as that in Embodiment 1 through the chemical vapor depositionmethod under the same condition as in Embodiment 1. The X-ray target isperformed with vacuum heat treatment at 2000° C. for 1 hour. The graindiameter of the fine structure tungsten-rhenium alloy at that time is 2to 8 μm. An actual load test was conducted using the above X-ray tube.As the result, it was confirmed that the X-ray target had an excellentload resistivity.

(Embodiment 4)

FIG. 15 is a cross-sectional view showing the construction of anotherembodiment of an X-ray target in accordance with the present invention.A fine structure tungsten-rhenium alloy 6 is coated onto the electronnon-irradiating surface of a molybdenum base plate 2 through thechemical vapor deposition method as the same manner as in Embodiment 1.The X-ray target is performed with the same vacuum heat treatment as inEmbodiment 1. An actual load test was conducted using the above X-raytube. As the result, it was confirmed that the X-ray target had anexcellent load resistivity.

(Embodiment 5)

Heat resistance of a target in accordance with the present invention wasstudied by a heating test. The target was manufactured in the samemanner as in Embodiment 1. A sintered tungsten-rhenium alloy having acoarse crystal grain diameter was laminated onto a molybdenum baseplate, and above it a fine structure tungsten-rhenium alloy was coatedthrough the chemical vapor deposition method, and then vacuum heattreatment was performed. From the result of the heating test using thetarget, coarsening due to crystal growth of the fine structuretungsten-rhenium alloy did not observed even in the very high heatingtemperature of 2000° C. FIG. 16 is a schematic cross-sectional viewshowing the crystal structure. It can be understood from FIG. 16 thatthe chemical vapor deposited tungsten-rhenium alloy having a finestructure formed on the base plate of the sintered tungsten-rheniumalloy having a coarse structure does not show any crystal growth andmaintains the fine structure after the heating test. Further, ananalysis by an X-ray method was performed to analyze residual stress inthe surface of the fine structure tungsten-rhenium alloy formed throughthe chemical vapor deposition method on the sintered tungsten-rheniumalloy base plate after the heating test. The result showed that acompressed stress existed at any temperature and accordingly there was astress field in which occurrence of crack due to heat load wassuppressed.

(Embodiment 6)

A mixed powder of tungsten powder and rhenium powder is mixed by a ballmixer, and tungsten powder is additionally added to the mixed powder andthe mixture is mixed using a V-type mixer for one hour. Paraffin isadded to the mixed powder as a binder and the mixed powder is dried byheating it in a vacuum environment. The dried powder is sifted through asieve to be classified. The classified powder is filled in a stampingdie having diameter of 100 mm, and molybdenum powder is filled above thefilled powder and then the powders are pressed with pressure of 300 MPato form a pressed powder body. The paraffin in the pressed powder bodyis burned by heating in a hydrogen flow and the pressed powder body issintered to form a sintered body. The sintered body obtained in such amanner is forged, cut and shaped to form a metal base plate for an X-raytarget. A film is formed on the electron irradiating surface of themetal base plate obtained in such a manner through the chemical vapordeposition method.

The film forming is performed by heating the metal base plate at 450° C.in a hydrogen gas environment, then introducing a mixed gas containingWF₆ onto the base plate. The base plate except the electron irradiatingsurface is masked with a graphite mask and the base plate is rotatedwith nearly 10 rpm during performing vapor deposition in order touniformly coating the circular periphery of the base plate. The chemicalvapor deposition is performed by controlling chemical vapor depositiontime so that film thickness of the tungsten thin film becomesapproximately 20μm. Then, a mixed gas added ReF₆ gas to WF₆ gas isintroduced onto the base pale to form a tungsten-rhenium thin film. Thefilm thickness is approximately 100 μm. The X-ray target manufactured insuch a manner is performed with vacuum heat treatment at 1400° C. for 1hour.

The grain diameter of the tungsten-rhenium alloy at that time is 0.9 to4.5 μm. Then, the target is assembled into a rotating anode andvacuum-sealed in an X-ray tube having a structure shown in FIG. 10.

(Embodiment 7)

A film is formed onto the electron irradiating surface of the metal baseplate manufactured in Embodiment 6 through the chemical vapor depositionmethod. The film forming is performed by heating the metal base plate at450° C. in a hydrogen gas environment, then introducing a mixed gascontaining WF₆ onto the base plate by controlling chemical vapordeposition time so that film thickness of the tungsten thin film becomesapproximately 10 μm. The base plate except the electron irradiatingsurface is masked with a graphite mask and the base plate is rotatedwith nearly 10 rpm during performing vapor deposition in order touniformly coating the circular periphery of the base plate as the sameas in Embodiment 6. Then, a mixed gas formed by adding a small amount ofReF₆ gas to WF₆ gas is introducing onto the base plate to form atungsten-rhenium thin film containing a small amount of rhenium. Afterthat, gradually increasing the adding amount of the ReF₆ gas isgradually increased so that the rhenium content at the electronirradiating surface becomes approximately 29 wt %. The total filmthickness is approximately 100 μm. The X-ray target manufactured in sucha manner is performed with vacuum heat treatment at 1400° C. for 1 hour.

The grain diameter of the tungsten-rhenium alloy at that time is 0.9 to4.5 μm. Then, the target is assembled into a rotating anode andvacuum-sealed in an X-ray tube having a structure shown in FIG. 10.

(Embodiment 8)

A film is formed onto the electron irradiating surface of the metal baseplate manufactured in Embodiment 6 through the chemical vapor depositionmethod. The chemical vapor deposition method is performed by introducinga mixed gas containing WF₆ and ReF₆ onto the base plate. The base plateexcept the electron irradiating surface is masked with a graphite maskand the base plate is rotated with nearly 10 rpm during performing vapordeposition in order to uniformly coating the circular periphery of thebase plate. Two kinds of X-ray targets are manufactured, that is, one isa target manufactured by stopping introducing both of the WF₆ gas andthe ReF₆ gas at a time in the middle of the chemical vapor depositionand the other is a target manufactured by stopping introducing only theWF₆ gas in the middle of the chemical vapor deposition. FIG. 17 isschematic views showing the multi-layer structures. FIG. 17 (a) showsthe multi-layer structure formed by stopping introducing both of the WF₆gas and the ReF₆ gas at a time in the middle of the chemical vapordeposition, and FIG. 17(b) shows the multi-layer structure formed bystopping introducing only the WF₆ gas in the middle of the chemicalvapor deposition. Since crystal growth is stopped for a while bystopping of introduction of gases, the x-ray generating metal layer isformed in a multi-layer structure having a layer boundary. In the X-raygenerating metal layer having such a structure, a crack once produced onthe surface does not reach the metal base plate immediately. The reasonis that progress of the crack is clinched. Thereby, there is very smallpossibility that a crack reaches the metal base plate immediately tocause peeling of the X-ray generating metal layer. The total filmthickness of the X-ray generating metal layer manufactured in such amanner is approximately 100 μm. The X-ray target manufactured in such amanner is performed with vacuum heat treatment at 1400° C. for 1 hour.The grain diameter of the tungsten-rhenium alloy at that time is 0.9 to4.5 μm. Then, the target is assembled into a rotating anode andvacuum-sealed in an X-ray tube having a structure shown in FIG. 10.

The X-ray target described above in accordance with the presentinvention has a high heat resistance since the electron irradiatingsurface is coated by the fine structure tungsten-rhenium alloy.Therefore, the X-ray tube incorporating the X-ray target in accordancewith the present invention can provide a highly bright medicalinspection image of CT apparatus since the X-ray tube can withstand asmall focus point and a high load.

What is claimed is:
 1. An X-ray tube comprising an anode target, theanode targe including: a metallic base body; and an X-ray generatingmetallic layer, formed on a surface of the metallic base body, thatgenerates X-rays upon irradiation with an electron beam; wherein theX-ray generating metallic layer includes a W-Re (tungsten-rhenium) alloylayer having a grain size of 0.9 μm to 10 μm and a thickness of 200 μmor less in at least a surface region of the X-ray generating metalliclayer that is to be irradiated with the electron beam.
 2. An X-ray tubeaccording to claim 1, wherein a W (tungsten) content in a portion of theW-Re (tungsten-rhenium) alloy layer that is in contact with the metallicbase body is higher than a W (tungsten) content in another portion ofthe W-Re (tungsten-rhenium) alloy layer that is to be irradiated withthe electron beam.
 3. An X-ray tube according to claim 2, wherein themetallic base body is any one of a base body including a Mo (molybdenum)base plate, a base body including a Mo (molybdenum) base plate and asintered W-Re (tungsten-rhenium) alloy layer formed on a surface of theMo (molybdenum) base plate to which the electron beam is to beirradiated, and a base body including a Mo (molybdenum) base plate, asintered W-Re (tungsten-rhenium) alloy layer formed on a surface of theMo (molybdenum) base plate to which the electron beam is to beirradiated, and graphite bonded to a surface of the Mo (molybdenum) baseplate to which the electron beam is not to be irradiated.
 4. An X-raytube according to claim 1, wherein the metallic base body is any one ofa base body including a Mo (molybdenum) base plate, a base bodyincluding a Mo (molybdemun) base plate and a sintered W-Re(tungsten-rhenium) alloy layer formed on a surface of the Mo(molybdenum) base plate to which the electron beam is to be irradiated,and a base body including a Mo (molybdenum) base plate, a sintered W-Re(tungsten-rhenium) alloy layer formed on a surface of the Mo(molybdenum) base plate to which the electron beam is to be irradiated,and graphite bonded to a surface of the Mo (molybdenum) base plate towhich the electron beam is not to be irradiated.
 5. An X-ray tubeaccording to claim 1, wherein the W-Re (tungsten-rhenium) alloy layerhas a grain size of 0.9 μm to 8 μm and a thickness of 200 μm or less inat least the surface region of the X-ray generating metallic layer thatis to be irradiated with the electron beam.
 6. An X-ray tube accordingto claim 1, wherein the W-Re (tungsten-rhenium) alloy layer has a grainsize of 0.9 μm to 4.5 μm and a thickness of 200 μm or less in at leastthe surface region of the X-ray generating metallic layer that is to beirradiated with the electron beam.
 7. A method of manufacturing an X-raytube including an anode target, the anode target including a metallicbase body, and an X-ray generating metallic layer, formed on a surfaceof the metallic base body, that generates X-rays upon irradiation withan electron beam, the method comprising the process of maintaining themetallic base body at a temperature in a range of 250° C. to 600° C. toform the X-ray generating metallic layer on the surface of the metallicbase body with a thickness of 200 μm or less composed of particleshaving a grain size from 0.9 μm to 10 μm using a CVD method that reducesa gas containing tungsten halide with hydrogen gas followed by heattreatment at a temperature in a range of 1000° C. to 2000° C.
 8. Amethod according to claim 7, wherein the particles have a grain size of0.9 μm to 8 μm.
 9. A method according to claim 7, wherein the particleshave a grain size of 0.9 μm to 4.5 μm.
 10. An X-ray tube comprising ananode target, the anode targe including: a metallic base body; and anX-ray generating metallic layer, formed on a surface of the metallicbase body, that generates X-rays upon irradiation with an electron beam;wherein the X-ray generating metallic layer includes a W-Re(tungsten-rhenium) alloy layer having a grain size of 0.9 μm to 10 μm inat least a surface region of the X-ray generating metallic layer that isto be irradiated with the electron beam.
 11. An X-ray tube according toclaim 10, wherein a W (tungsten) content in a portion of the W-Re(tungsten-rhenium) alloy layer that is in contact with the metallic basebody is higher than a W (tungsten) content in another portion of theW-Re (tungsten-rhenium) alloy layer that is to be irradiated with theelectron beam.
 12. An X-ray tube according to claim 11, wherein themetallic base body is any one of a base body including a Mo (molybdenum)base plate, a base body including a Mo (molybdenum) base plate and asintered W-Re (tungsten-rhenium) alloy layer formed on a surface of theMo (molybdenum) base plate to which the electron beam is to beirradiated, and a base body including a Mo (molybdenum) base plate, asintered W-Re (tungsten-rhenium) alloy layer formed on a surface of theMo (molybdenum) base plate to which the electron beam is to beirradiated, and graphite bonded to a surface of the Mo (molybdenum) baseplate to which the electron beam is not to be irradiated.
 13. An X-raytube according to claim 10, wherein the metallic base body is any one ofa base body including a Mo (molybdenum) base plate, a base bodyincluding a Mo (molybdemun) base plate and a sintered W-Re(tungsten-rhenium) alloy layer formed on a surface of the Mo(molybdenum) base plate to which the electron beam is to be irradiated,and a base body including a Mo (molybdenum) base plate, a sintered W-Re(tungsten-rhenium) alloy layer formed on a surface of the Mo(molybdenum) base plate to which the electron beam is to be irradiated,and graphite bonded to a surface of the Mo (molybdenum) base plate towhich the electron beam is not to be irradiated.
 14. An X-ray tubeaccording to claim 10, wherein the W-Re (tungsten-rhenium) alloy layerhas a grain size of 0.9 μm to 8 μm in at least the surface region of theX-ray generating metallic layer that is to be irradiated with theelectron beam.
 15. An X-ray tube according to claim 10, wherein the W-Re(tungsten-rhenium) alloy layer has a grain size of 0.9 μm to 4.5 μm inat least the surface region of the X-ray generating metallic layer thatis to be irradiated with the electron beam.
 16. A method ofmanufacturing an X-ray tube including an anode target, the anode targetincluding a metallic base body, and an X-ray generating metallic layer,formed on a surface of the metallic base body, that generates X-raysupon irradiation with an electron beam, the method comprising theprocess of maintaining the metallic base body at a temperature in arange of 250° C. to 600° C. to form the X-ray generating metallic layeron the surface of the metallic base body composed of particles having agrain size from 0.9 μm to 10 μm using a CVD method that reduces a gascontaining tungsten halide with hydrogen gas followed by heat treatmentat a temperature in a range of 1000° C. to 2000° C.
 17. A methodaccording to claim 16, wherein the particles have a grain size of 0.9 μmto 8 μm.
 18. A method according to claim 16, wherein the particles havea grain size of 0.9 μm to 4.5 μm.